How can satellites 20,000 km away locate an animal?

POSITIONING

How can satellites 20,000 km away locate an animal?

A cow is standing somewhere in a pasture. A small battery-powered device on its collar listens to radio signals transmitted by satellites orbiting roughly 20,000 kilometres above the Earth. From these incredibly weak signals, the device can work out where the cow is, often to within just a few metres.

Perhaps even more remarkably, the tag does not communicate with the satellites at all. It only listens.

Measuring distance by measuring time

The basic idea behind satellite positioning is surprisingly simple. Satellites continuously transmit radio signals containing extremely accurate timing information and information about their orbits. Since radio waves travel at the speed of light, a receiver can use the arrival times of signals from several satellites to determine its position.

The numbers involved are extraordinary. Light travels approximately 300,000 kilometres in one second. That means it travels about 300 kilometres in a millisecond, 300 metres in a microsecond – and just 30 centimetres in a nanosecond.

So if you want to measure distances accurately using radio signals, you need very good clocks.

This is one reason GNSS satellites carry extremely precise atomic clocks. By listening to several satellites simultaneously, a receiver can solve for its three-dimensional position and correct for the error in its own much simpler clock. As a useful side effect, GNSS therefore provides not only position but also extremely accurate time.

GPS is only part of the story

We commonly call satellite positioning “GPS”, but GPS is actually the American satellite navigation system. It was the first global system to become widely used, and the name stuck – much as we use “Google” as a verb for searching the Internet.

The more general term is GNSS – Global Navigation Satellite System.

Today there are four major global systems: the American GPS, Europe’s Galileo, China’s BeiDou and Russia’s GLONASS. Each operates its own constellation of satellites, and modern receivers can use several constellations simultaneously. Japan and India also operate regional satellite navigation systems.

This means that the tiny receiver in a modern phone or animal tag can listen to signals from satellites belonging to several completely independent systems.

20,000 kilometres is a long way

GNSS satellites orbit at around 20,000 kilometres above the Earth’s surface – more than the Earth’s own diameter. For comparison, low-Earth-orbit communication satellites such as Starlink fly only a few hundred kilometres above us.

By the time a GNSS signal reaches the ground, it is extremely weak. The receiver must pick it out from background radio noise and determine its timing with remarkable precision.

This is why satellite positioning works beautifully under an open sky but becomes more difficult around buildings, beneath dense vegetation or when the antenna has a poor view of the sky. Signals can also bounce from nearby objects before reaching the receiver, creating additional errors.

Under good conditions, an ordinary modern GNSS receiver can typically determine its position within a few metres. With specialised equipment and correction techniques such as RTK, the same fundamental technology can achieve centimetre-level accuracy.

So why don’t we put survey-grade positioning into every animal tag?

Accuracy has a price

For a battery-powered device, the important question is not simply how accurately can we determine the position?

The better question is:

How accurately and how often do we need to know the position to solve the problem?

A surveyor may need centimetres. Someone trying to find a cow in a large grazing area usually does not.

There is another important difference. A survey instrument can have a substantial power supply and be recharged frequently. An animal tag should ideally operate unattended for a very long time.

Obtaining GNSS positions consumes energy. Taking positions more frequently gives us a more detailed picture of movement, but also reduces battery life. Demanding better positioning under difficult conditions can require the receiver to remain active for longer, consuming still more energy.

Designing a good tracking system therefore involves compromises between positioning accuracy, update frequency, communication and battery life. Maximum accuracy is not necessarily the best engineering solution.

From coordinates to useful information

At Qulinda, a GNSS position is not the final product. It is one piece of information in a much larger system.

A sequence of positions can tell us how an animal moves through a landscape. Positions can be related to farms, grazing areas, water sources or protected zones. The system can detect when an animal enters or leaves an area and decide when information needs to be communicated to the user.

The challenge is therefore not simply to put a GPS receiver on an animal. It is to design the complete system around what the user actually needs to know – while making the hardware practical enough to operate for long periods in the real world.

And all of this starts with a tiny receiver listening to clocks flying 20,000 kilometres above the Earth.